Sintered ceramic, ceramic sphere, and device for inspecting ceramic sphere

ABSTRACT

Provided are a sintered ceramic and a ceramic sphere which are inhibited from suffering surface peeling due to fatigue resulting from repetitions of loading and can attain an improvement in dimensional accuracy when subjected to surface processing and which have excellent wear resistance and durability. A ceramic-sphere inspection device is also provided with which a ceramic sphere is inspected for a flaw present in the surface layer and for snow flakes without destroying the ceramic sphere. The device is a ceramic-sphere inspection device ( 100 ) in which a ceramic sphere (S) is rotatably supported in a given position and illuminating light emitted from a light projector ( 110 ) is detected with a light receiver ( 120 ) to evaluate the state of the inner part of the surface layer, and has been configured so that the light receiver ( 120 ) does not detect the light emitted from the light projector ( 110 ) and reflected at the surface of the ceramic sphere.

TECHNICAL FIELD

The present invention relates to a sintered ceramic, a ceramic sphereand a ceramic-sphere inspection device which have excellent wearresistance and durability, and, more particularly, relates to a sinteredceramic and a ceramic sphere which are suitable to apply to a rollingmember used in a sliding device such as a spherical, columnar, circulartruncated conical, barrel-shaped or hourglass bearing or a ball nut, ora valve body and the like of a fluidic valve which controls a highpressure fluid, and a ceramic-sphere inspection device which is suitableto inspect a state of an inner part of a surface layer of this ceramicsphere.

BACKGROUND ART

Although a sintered ceramic requires higher manufacturing cost thanmetal such as steel, it has high mechanical strength and excellent wearresistance and rigidity, has a lighter specific weight than steel andhas an insulating property and high corrosion resistance.

By utilizing these characteristics, and being used as a wear resistantmember for a sliding device such as a bearing or a ball nut, or a valveand the like of a fluidic valve which controls a high pressure fluid, itis possible to reduce the weight, prevent damages due to a load andrepetitions of sliding or damages such as wear, corrosion and electricalcorrosion, maintain performance for a long period of time, increase thelife of components and reduce maintenance labor.

For bearings and the like which are used in an environment particularlyin the vicinity of wind power generators, compressors ofair-conditioners, vehicles and the like of electrical vehicles andhybrid vehicles and electrical systems, and in which the temperature andhumidity significantly change, ceramic spheres are frequently employedinstead of metals such as steel which is significantly influenced bydamages and the like by corrosion or electric corrosion and requireslower manufacturing cost, since the ceramic spheres require lowermaintenance cost than metals such as steel.

Further, a rigid, light weight and long-life valve body is required fora fluidic valve in particular which is opened and closed at a high speedunder a high pressure, and therefore employing a ceramic sphere for thevalve body brings a significant advantage.

A general ceramic is sintered using a plurality of raw materials andsintering agents and, when, for example, a silicon nitride sinteredcompact is sintered, silicon nitride (Si₃N₄) which is a raw materialhardly causes solid-state sintering itself and cannot provide a densesintered compact, and therefore a rare-earth oxide such as Y₂O₃ and anoxide such as Al₂O₃ are mixed and molded as sintering agents and aredensified by liquid-phase sintering to obtain a silicon nitride sinteredcompact.

Thus, when a ceramic is sintered using a plurality of raw materials andsintering agents, fine flaws are produced on a surface or in an innerpart depending on conditions and these flaws cause surface peeling dueto fatigue resulting from repetitions of loading.

For example, known Patent Literature 1 discloses defining the porosityof a sintered compact and the maximum air hole diameter in a grainboundary phase, and obtaining a wear resistant member made of a siliconnitride sintered compact which has an excellent rolling life becauseflaws such as scratches and cracks on a rolling element surface lead todeterioration of reliability of quality.

However, the technique disclosed in Patent Literature 1 only targets atflaws such as scratches, the cracks and the air holes, and does notprovide any countermeasure for white spots (snow flakes) which influencemore on wear resistance and durability than on the scratches, cracks andair holes.

Further, known Patent Literature 2 discloses focusing on the compositionwhich is observed as white branches formed with an aggregate ofmicropores (fine air holes corresponding to fine flaws) in a range offrom a surface to the depth of 1 mm, and, if the aggregate of themicropores has a given size or less, causing peeling due to rollingfatigue which causes a trouble when a ball is used for a bearingmaterial, irrespective of the rate of an area which occupies in theentire area of the ball.

However, although the aggregate of micropores has a small size, if theamount of micropores is great, a crushing load decreases, therebycausing damages and causing surface peeling due to fatigue resultingfrom repetitions of loading.

Further, this white branch portion has different characteristics such asthe rigidity and the density from other portions, and therefore, even asmall white branch portion blocks improvement in dimensional accuracysuch as the sphericity and the surface roughness when a silicon nitridesphere or a silicon nitride roller which is a wear resistant member ispolished and processed, resulting in causing surface peeling due tofatigue resulting from repetitions of loading.

Furthermore, there are problems that flaws or remaining internaldistortion inside a silicon nitride sphere or a silicon nitride rollerwhich is a wear resistant member make an internal stress state unevenand become starting points of destruction, or cause wear or vibrationbecause dimensional accuracy does not improve as described above.

Still further, when a ceramic sphere is inspected as a product, a meansis required for observing and inspecting that there are no flaws such asscratches, cracks and air holes near a surface nor snow flakes withoutdestroying the ceramic sphere.

As a device which inspects a ceramic sphere without destroying theceramic sphere, a device which optically observes the surface of theceramic sphere as disclosed in, for example, Patent Literature 3 and adevice which observes the surface of the ceramic sphere and the innerpart of the surface layer by means of an ultrasonic wave as disclosed inPatent Literature 4 are known.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2002-326875 (all pages and FIG. 2)

Patent Literature 2: JP-A No. 6-329472 (all pages and FIG. 1)

Patent Literature 3:,JP-A No. 2008-51619 (all pages and FIGS. 1 and 3)

Patent Literature 4: JP-A No. 2010-127621 (all pages and FIG. 2)

SUMMARY OF INVENTION Technical Problem

As a result of a devoted study in light of the above current situation,the inventors found that it is possible to obtain a sintered ceramicwhich has excellent wear resistance and durability and solves the aboveproblem, by limiting the composition of sintering agents, adjusting thebulk density and the average grain size in a given range, controllingthe composition from the surface to a given depth and manufacturing thesintered ceramic under limited conditions.

The conventional techniques are supposed to deal with flaws such asscratches, cracks and air holes which cause peeling due to rollingfatigue as disclosed in the above Patent Literatures.

However, as a result of the devoted study, the inventors found that onlyreducing these flaws as much as possible is not sufficient forimprovement in wear resistance and durability.

That is, the inventors found that, although these flaws can be observedby a scanning electron microscope (SEM), other flaws cannot be observedby the SEM, and the presence of white spots (snow flakes) which isobserved by an optical microscope significantly influences wearresistance and durability.

While these snow flakes are not observed at all by the SEM observationas illustrated in FIG. 2, the snow flakes are clearly observed by theoptical microscope.

These snow flakes are not observed by the SEM observation, are not whitebranches made of an aggregate of micropores as disclosed in PatentLiterature 2 and are supposed to have a slight difference in thecomposition of the crystal grain boundary phase from other portions, andthis slight difference in the composition significantly influences wearresistance and durability.

Further, the inventors found that, when a sintered ceramic is used for awear resistant member, not only flaws such as scratches, cracks and airholes on the surface of the member and near the surface, but also snowflakes significantly influence wear resistance and durability andtherefore whether or not there are the flaws and the snow flakes on thesurface and near the surface matters, and it is possible to provide thecomposition without the flaws and the snow flakes from the surface tothe depth of 250 μm by limiting the composition of a sintering agent,adjusting the bulk density and the average grain size in a given rangeand manufacturing the sintered ceramic under limited conditions.

It is therefore an object of the present invention to provide a sinteredceramic and a ceramic sphere which can reduce surface peeling due tofatigue resulting from repetitions of loading and attain improvement indimensional accuracy upon process of the surface, and has excellent wearresistance and durability.

Further, when the ceramic sphere is inspected as a product taking intoaccount characteristics found by the inventors, a means is required forobserving and inspecting that there are not defects such as scratches,cracks and air holes near the surface nor snow flakes without destroyingthe ceramic sphere.

However, although a device which performs optical observation as in theabove Patent Literature 3 detects light reflected from the surface andtherefore can detect flaws appeared on the surface and a difference inthe color tone such as snow flakes, this device has had a problem thatthe device cannot observe flaws, snow flakes and the like in the innerpart of the surface layer which do not appear on the surface.

Further, although a device which performs observation using anultrasonic wave as in Patent Literature 4 can detect, for example,scratches, cracks and air holes of varying reflections of ultrasonicwaves in the inner part of the surface layer, the device has difficultyin detecting snow flakes formed based on the above slight difference inthe composition of the crystal grain boundary phase, based onreflections of ultrasonic wave.

It is therefore another object of the present invention to provide aceramic-sphere inspection device which detects whether or not there areflaws and snow flakes in the inner part of the surface layer withoutdestroying a ceramic sphere with a simple configuration.

Solution to Problem

To solve the above problem, the invention according to claim 1 providesa sintered ceramic which is obtained by molding and sintering a mixtureincluding silicon nitride and a sintering agent made of Al₂O₃ and Y₂O₃,wherein a bulk density is 3.1 g/cm³ or more, an average grain size is 3μm or less, and there are no flaw of 10 μm or more from a surface to adepth of 250 μm and no white spot (snow flake) of 20 μm or more.

To solve the above problem, the invention according to claim 2 providesa ceramic sphere which is obtained by molding and sintering in aspherical shape a mixture including silicon nitride and a sinteringagent made of Al₂O₃ and Y₂O₃, wherein a bulk density is 3.1 g/cm³ ormore, an average grain size is 3 μm or less, and there are no flaw of 10μm or more from a surface to a depth of 250 μm and no white spot (snowflake) of 20 μm or more.

To solve the above problem, the invention according to claim 3 providesa wear resistant member formed using a sintered ceramic, wherein asurface of the sintered ceramic according to claim 1 is processed tomake a rolling bearing member.

To solve the above problem, the invention according to claim 4 providesa ceramic-sphere inspection device including a rotation supporter whichrotatably supports the ceramic sphere according to claim 2, at a givenposition, a light projector which emits illuminating light toward asurface of the ceramic sphere, a light receiver which detects lightreflected from the ceramic sphere as inspection light, and a processorwhich evaluates a state of an inner part of a surface layer of theceramic sphere in'response to a detection output from the lightreceiver, wherein the light receiver does not detect the illuminatinglight emitted from the light projector and reflected at the surface ofthe ceramic sphere.

To solve the above problem, the invention according to claim 5 providesthe ceramic-sphere inspection device with further configurationsaccording to claim 4, wherein the light projector includes a lightsource, and a light projecting unit which guides light of the lightsource to the surface of the ceramic sphere as illuminating light, thelight receiver includes a light amount detecting unit, and a lightreceiving unit which guides the inspection light from the ceramic sphereto the light amount detecting unit, and at least one of the lightprojecting unit and the light receiving unit includes at a front end acontact surface which can contact the surface of the ceramic sphere.

To solve the above problem, the invention according to claim 6 providesthe ceramic-sphere inspection device with further configurationsaccording to claim 4 or claim 5, wherein a plurality of light receivingunits are provided over a semicircle of an outer peripheral circle in across section passing a center of the ceramic sphere, and the rotationsupporter rotates the ceramic sphere at a right angle with respect tothe outer peripheral circle on which the light receiving units areprovided.

To solve the above problem, the invention according to claim 7 providesthe ceramic-sphere inspection device with further configurationsaccording to claim 4 or claim 5, wherein a plurality of light receivingunits are provided along part of an outer peripheral circle in a crosssection passing a center of the ceramic sphere and the rotationsupporter rotates the ceramic sphere at a right angle with respect tothe outer peripheral circle on which the light receiving units areprovided, and rotates the ceramic sphere in an outer peripheral circledirection on which the light receiving units are provided such that theceramic sphere is shifted by a width of the plurality of light receivingunits when finishing rotating round.

To solve the above problem, the invention according to claim 8 providesthe ceramic-sphere inspection device with further configurationsaccording to claim 4 or claim 5, wherein only one light receiving unitis provided, and the rotation supporter rotates the ceramic sphere in agiven direction, and slightly rotates the ceramic sphere at a rightangle with respect to a rotation direction.

To solve the above problem, the invention according to claim 9 providesthe ceramic-sphere inspection device with further configurationsaccording to one of claim 6 to claim 8, wherein a same number of lightprojecting unit as the number of the plurality of light receiving unitsis provided, and each one of the light projecting units is providedadjacent to each of the plurality of light receiving units.

To solve the above problem, the invention according to claim 10 providesthe ceramic-sphere inspection device with further configurationsaccording to one of claim 5 to claim 9, wherein the rotation supportercan be intermittently driven, at least one of the light projecting unitand the light receiving unit can proceed and retreat to and from asurface direction of the ceramic sphere, and a contact surface at afront end of at least one of the light projecting unit and the lightreceiving unit closely attaches to the surface of the ceramic spherewhen driving of the rotation supporter is stopped, and is detached fromthe surface of the ceramic sphere when the rotation supporter is driven.

ADVANTAGEOUS EFFECTS OF INVENTION

The sintered ceramic of the invention according to claim 1 and theceramic sphere of the invention according to claim 2 can attainimprovement in wear resistance and dimensional accuracy without fineflaws and remaining internal distortion and without causing surfacepeeling due to fatigue resulting from repetitions of loading and makingan uneven internal stress state and a starting point of destruction.

The wear resistant member of the invention according to claim 3 canimprove durability of a rolling bearing, and reduce wear and vibration.

With the ceramic-sphere inspection device of the invention according toclaim 4, the light receiver does not detect illuminating light emittedfrom the light projector and reflected at the surface of the ceramicsphere, and can detect as inspection light only illuminating lighttransmitted and diffused in the inner part of the surface layer of theceramic sphere and reflected from the inner part, so that it is possibleto accurately detect whether or not there are flaws in the inner part ofthe surface layer and snow flakes formed based on a slight difference inthe composition of the crystal grain boundary phase without destroyingthe ceramic sphere and without depending on the surface state.

Further, the optical transmittance is determined according to a materialand sintering conditions of the ceramic sphere, so that it is possibleto inspect whether or not the material of the ceramic sphere is good andthe ceramic sphere is sintered well by observing the total amount ofinspection light detected at a plurality of portions.

With the configuration according to claim 5, at least one of the lightprojecting unit and the light receiving unit has at the front end thecontact surface which can contact the surface of the ceramic sphere, sothat it is possible to reliably prevent illuminating light emitted fromthe light projecting unit and reflected at the surface of the ceramicsphere from being detected by the light receiving unit even when thelight projecting unit and the light receiving unit are close.

With the configuration according to claim 6, it is possible to observethe entire surface of the ceramic sphere and efficiently inspect theceramic sphere only by rotating the ceramic sphere once.

With the configuration according to claim 7, it is possible to reducethe number of light projecting units and the number of light receivingunits, simplify the entire device and reduce cost.

With the configuration according to claim 8, it is possible to reducethe number of light projecting units and the number of light receivingunits to one respectively, further simplify the entire device, reducecost, and the light amount and sensitivity of a plurality of lightprojecting units and light receiving units do not need to be adjusted atall and, consequently, it is easy to maintain inspection accuracy.

With the configuration according to claim 9, for example, the detectionsensitivity does not need to be adjusted based on the relationshipbetween positions of the light projecting unit and the light receivingunit and, consequently, it is easy to maintain inspection accuracy.

With the configuration according to claim 10, the contact surface at thefront end of at least one of the light projecting unit and the lightreceiving unit is not damaged or worn away due to sliding against thesurface of the ceramic sphere and, consequently, it is easy to maintaininspection accuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a halogen light transmission optical micrograph of a sinteredceramic according to the present invention.

FIG. 2 is a micrograph of a conventional sintered ceramic.

FIG. 3 is a halogen light transmission optical micrograph of aconventional sintered ceramic.

FIG. 4 is an explanatory view of measurement of a crushing load.

FIG. 5 illustrates sintering conditions of sintered ceramics accordingto the present invention and comparative examples.

FIG. 6 illustrates endurance test results of a bearing ball made of aceramic sphere according to the present invention and the comparativeexamples.

FIG. 7 illustrates two-sphere crushing load test results of a bearing ofa ⅜ inch standard according to the present invention and comparativeexamples.

FIG. 8 illustrates two-sphere crushing load test results of a bearingball of a 5/32 inch standard according to the present invention and thecomparative examples.

FIG. 9 is a schematic side view of a ceramic-sphere inspection deviceaccording to a first example of the present invention.

FIG. 10 is a schematic plan view of the ceramic-sphere inspection deviceaccording to the first example of the present invention.

FIG. 11 is a schematic plan view of a ceramic-sphere inspection deviceaccording to a second example of the present invention.

FIG. 12 is a schematic front view of the ceramic-sphere inspectiondevice according to the second example of the present invention.

FIG. 13 is a schematic plan view of a ceramic-sphere inspection deviceaccording to a third example of the present invention.

FIG. 14 is a schematic plan view of another embodiment of theceramic-sphere inspection device according to the third example of thepresent invention.

FIG. 15 is a schematic view of another embodiment of the ceramic-sphereinspection device according to the present invention.

FIG. 16 is a schematic view of a still another embodiment of theceramic-sphere inspection device according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, each requirement which need to be satisfied by a sinteredceramic according to the present invention which has excellent wearresistance will be described in detail.

The sintered ceramic according to the present invention needs to havethe bulk density of 3.1 g/cm³ or more and, preferably, have the bulkdensity of 3.2 g/cm³ or more.

When the bulk density is less than 3.1 g/cm³, there are multiplemicropores inside the sintered compact, and therefore resistance againstthe external stress such as wear and shock deteriorates, and wearresistance and durability decrease.

Further, the average grain size needs to be 3 μm or less and, morepreferably, is 2 μm or less.

When the average grain size exceeds 3 μm, the average grain sizeincreases and the crystal grain boundary phase area increases, and, whensnow flakes are formed in this crystal grain boundary, the flaw sizeincreases and remaining internal distortion becomes significant, therebycausing significant decrease in wear resistance and durability.

In addition, the average crystal grain size in the sintered ceramicaccording to the present invention is measured by mirror-finishing thesurface of the sintered compact using a diamond wheel and abrasivegrains and applying HF etching or plasma etching to the surface, and isobserved by a SEM at the magnification at which 100 or more averagegrain sizes can be observed in one field of view.

Silicon nitride crystal grains of the sintered ceramic according to thepresent invention are mainly columnar, and therefore, a long diameterand a short diameter of the crystal grains are measured to obtain thegrain diameter of one crystal grain based on grain diameter=(longdiameter+short diameter)/2.

Grain diameters of 100 crystal grains are obtained in this way to usethe average value of the 100 crystal grains as the average crystal grainsize.

Further, there should be no flaws of 10 μm or more from the surface tothe depth of 250 μm, and no snow flakes of 20 μm or more.

When there are flaws of 10 μm or more and snow flakes of 20 μm or more,the resistance against the external stress such as shock deterioratesand wear resistance and durability decrease, and, when there are noflaws of 5 μm or more and snow flakes of 10 μm or more, this is morepreferable.

The flaws include not only scratches, cracks and air holes, but alsoaggregation of a sintering agent and a second phase containingimpurities.

It is possible to evaluate whether or not there are flaws and snowflakes on the surface and near the surface by allowing transmission ofhalogen light having the wavelength between 500 nm and 800 nm in thesintered compact, and allow transmission of light from the surface tothe depth of 250 μm under observation conditions of the presentinvention.

FIG. 1 is an optical micrograph showing that the sintered ceramicaccording to the present invention is sliced to 0.2 mm of a sheetthickness and polished and halogen light having the wavelength between500 nm and 800 nm is transmitted, and that no snow flakes and flaws canbe observed.

Meanwhile, FIG. 3 is an optical micrograph showing that a conventionalsintered ceramic is sliced to 0.2 mm of a sheet thickness and polishedand halogen light having the wavelength between 500 nm and 800 nm istransmitted, and that snow flakes and flaws which are supposed to besegregation can be clearly observed.

When light does not transmit from the surface to the depth of 250 μm orwhen transmission light is uneven, this means that there are not onlyflaws in portions between the surface and the depth of 250 μm but alsosnow flakes of at least 20 μm or more.

Therefore, when halogen light does not transmit from the surface to thedepth of 250 μm or transmission light is uneven, wear resistance anddurability decrease.

In addition, with the present invention, flaws are measured by polishinga sintered compact to have a mirror surface using a diamond wheel andabrasive grains, observing ten sites at random at 1000 folds of themagnification by means of the SEM and using the observed maximum size ofthe flaws for the flaw size.

Further, snow flakes are measured in a similar manner by observing tensites at random in the mirror-polished sintered compact at 100 folds ofthe magnification by means of an optical microscope using halogen lighthaving the wavelength between 500 nm and 800 nm, and using the observedmaximum size of the snow flakes as the size of the snow flakes.

The sintered ceramic according to the present invention which hasexcellent wear resistance has high mechanical characteristics, and, forexample, a two-sphere crushing load against a bearing ball of the ⅜ inchstandard made of the sintered ceramic according to the present inventionand a SUJ2 ball is 100 kN or more and a two-sphere crushing load of thesame bearing balls is 20 kN or more.

Further, the sintered ceramic according to the present invention hasexcellent wear resistance, durability and mechanical characteristicsand, consequently, is useful as a rolling bearing member.

In addition, the crushing load is measured by applying the load at aspeed of 2 to 6 kN/s (204 to 612 kgf/s) (a method based on previousJISB1501) in a state where two measurement target spheres S1 and S2 ofthe same size are vertically aligned by anvils 510 and 520 providedabove and below S1 and S2 and having hardness of HRC60 or more andconical seats at an angle of 120° C., as shown in FIG. 4.

Measurement performed together with the ceramic sphere and a SUJ2 steelball is performed by arranging the ceramic sphere above and the SUJ2steel ball below.

A method of manufacturing the sintered ceramic according to the presentinvention will be described below.

In addition, the following manufacturing method is an example, and thesintered ceramic and the ceramic sphere according to the presentinvention are by no means limited by this manufacturing method.

Silicon nitride powder to use includes an a phase of 80% or more and,more preferably, includes the a phase of 90% or more.

The purity needs to be 98% or more and, more preferably, 98.5% or more.

When impurities exceed 2%, multiple phases 2 containing the impuritiesare formed inside the sintered compact.

The specific surface area is 6 m²/g or more and, more preferably, 8 m²/gor more.

When the specific surface area is less than 6 m²/g, sinteringperformance decreases.

Alumina (Al₂O₃) and yttria (Y₂O₃) are added as sintering agents withsilicon nitride powder having the above characteristics.

By adding the amount of 3 to 6 wt % and, more preferably, 4 to 6 wt % ofalumina and yttria, it is possible to not only provide excellent wearresistance but also to provide the composition which allows transmissionof halogen of 500 nm to 800 nm.

Alumina to use needs to have the purity of 99% or more and, morepreferably, 99.5% or more, and the specific surface area needs to be 6m²/g or more and, more preferably, 7 m²/g or more.

Further, the purity of yttria needs to be 99% or more and, morepreferably, 99.5% or more, and the specific surface area needs to be 8m²/g or more and, more preferably, 9 m²/g or more.

When the purities of alumina and yttria powder do not satisfy definedvalues, the amount of impurities increases similarly to a case where thepurity of silicon nitride powder is less than the defined value.

Further, when the specific surface area of powder does not satisfy adefined value, it is hardly diffused evenly in the silicon nitridepowder, and an aggregate of sintering agents is formed in the sinteredcompact and unevenness in the composition of the glass phase increases,thereby producing snow flakes.

A medium agitating mill is preferably used instead of a mill such as aball mill to mix and evenly diffuse silicon nitride powder, alumina andyttria powder.

Molding powder is obtained by adding a binder of a given amount to theresulting evenly-mixed powder and SD-drying the powder.

The specific surface area of the molding powder is 10 to 15 m²/g and,more preferably, 10 to 13 m²/g.

While sintering performance is low when the specific surface area ofmolding powder is less than 10 m²/g, powder becomes fine when thespecific surface area exceeds 15 m²/g, molding pressure transmissibilityupon molding decreases, multiple flaws are formed inside the compact andmultiple flaws are formed inside the sintered compact after sintering.

The molding powder is molded in a given shape using CIP (Cold IsostaticPress) molding to obtain an even compact.

A molding pressure works only in one direction according to a pressmolding method using a mold, the density difference between an innerpart and an external part of the compact becomes significant, and theresidual stress is produced in the sintered compact due to a sinteringshrinkage difference, thereby producing, for example, cracks and leadingto flaws in the compact.

The resulting compact is degreased and is then sintered. The temperatureof heating the compact in a sintering container made of silicon nitrideis increased from the room temperature to 1000 to 1250° C. in vacuum of10⁻² Pa or less, and the compact is then sintered at 1600 to 1850° C.and, more preferably, at 1600 to 1800° C. in a nitrogen atmosphere.

Heating in this vacuum is directed to removing components whichnegatively influence sintering performance vaporizing from the compactdue to heating, and removing a hydroxyl group or oxygen adsorbed to thesurface of silicon nitrogen powder due to, for example, raw materialprocessing.

When heating is performed in vacuum at a temperature less than 1000° C.,this effect does not work and, when the heating is performed to atemperature more than 1250° C. in vacuum, the discharged amount of, forexample, oxygen contained in the compact increases, and, as a result,the amount of the glass phase formed in the sintered compact decreasesand flaws such as air holes increase in the crystal grain boundary.

Further, when the sintering temperature is less than 1600° C., sinteringis not promoted and, when the sintering temperature exceeds 1850° C.,dissolution of silicon nitride is promoted, thereby forming, forexample, multiple flaws and decreasing mechanical restriction.

In addition, the compact is put in the sintering container made ofsilicon nitride and sintered.

When a sintering container made of carbon is used, carbon gas isproduced at 1500° C. or more and solid-soluted in the compact which isbeing sintered, the color tone increases, and not only halogen light isnot transmitted but also flaws on the surface and near the surfaceincrease and mechanical characteristics decrease.

Further, by performing HIP processing of the resulting sintered compact,it is possible to obtain a high quality sintered compact with littleflaws. In addition, HIP processing conditions include that HIPprocessing is performed at 1500 to 1700° C. and, more preferably, at1550 to 1700° C. at a gas pressure of 100 MPa or more.

Although the resulting sintered ceramic is processed to a given size tomake a wear resistant member, the sintered ceramic according to thepresent invention includes no flaws and no snow flakes, so that it ispossible to increase process accuracy and surface roughness.

Consequently, when the ceramic sphere is used as a bearing ball, it iseasy to make the sphericity of 0.05 μm or less and surface roughness(Rmax) of 0.01 μm or less.

As examples and comparative examples according to the present invention,a sintered ceramic was sintered under each condition illustrated in FIG.5 to make a bearing ball of the ⅜ inch standard.

These bearing balls were made by wet pulverizing and mixing siliconnitride powder, alumina and yttria powder, adding wax emulsion of 3% byweight of the powder weight to the resulting mixed slurry, performingspray-drying and performing CIP molding at the pressure of 100 MPa usinga rubber mold.

Further, the resulting compact was degreased at 400° C. in theatmosphere, put in the sintering container made of silicon nitride,heated in vacuum of 10⁻² Pa from the room temperature to 1100° C. andsintered for four hours at 1560 to 1880° C. in the nitrogen atmosphereto make a ball of φ10 mm.

In addition, in Comparative Example 3, the compact was heated in vacuumof <10⁻² Pa from the room temperature to 1350° C. and was sintered and,in Comparative Example 2, the compact was heated in vacuum of <10⁻² Pafrom the room temperature to 800° C. and was sintered.

Further, in Comparative Example 9, the compact was sintered using thesintering container made of carbon upon sintering.

The balls made were polished and processed to make bearing balls of the⅜ inch standard.

FIG. 6 illustrates characteristics and results of fine structureobservation and endurance of these bearing balls.

In addition, the endurance test was conducted by embedding a bearingball in Brg6206 (9/Brg), and using a Gakushin-type life tester (purerolling) at 2000 rpm of the number of primary axis rotations with 850kgf of a load (pure radial load) and using clean oil (60° C.) forsmoothness, and, if rotation continued for 1300 h without abnormality,the test was finished.

FIG. 7 illustrates two-sphere crushing load test results of bearingballs of the ⅜ inch standard made of the sintered ceramic according toone embodiment of the present invention obtained as described above.

Samples used for experiments include samples illustrated in FIGS. 5 and6 according to Example 5 of the present invention and examplesillustrated in FIGS. 5 and 6 according to Comparative examples 4 and 6,and the samples of the same manufacturing lot are used for examples andcomparison examples.

Further, the speed to add a crushing load was 6 kN/s (612 kgf/s).

Although, with the bearing ball according to Comparative Example 4, upontwo-sphere crushing of two bearing balls, a crushing load is roughly 20kN or more and the crushing strength is good and stable, upon two-spherecrushing against a steel ball (SUJ-2 ball), a crushing load is 67.7 to115.7 kN, the crushing strength significantly varies including a lowstrength and characteristics are not stable.

While, with the bearing ball according to Comparative Example 6, upontwo-sphere crushing against a steel ball (SUJ-2 ball), a crushing loadis 78.5 to 112.8 kN and has variation less than in Conventional Example1 and varies including a little crushing strength, and, even upontwo-sphere crushing of bearing balls, the crushing load is 13.7 to 19.2kN, the crushing strength is low and varies, and characteristics are notstable.

With the bearing balls according to Comparative Examples 4 and 6, snowflakes in the bearing balls make an irregular inner stress state unevenand become a starting point of destruction, internal distortion remains,the amount of an agent exceeding a given amount and a sinteringtemperature higher than a given temperature increases a thermalexpansion difference between silicon nitride crystal and the glass phaseand increases internal distortion, and unevenness in the compositionincreases. Therefore it is not possible to stably secure the crushingstrength and it is difficult to make characteristics of multiple bearingballs even.

In contrast, with the bearing ball according to Example 5, upontwo-sphere crushing of bearing balls, the crushing load is 20 kN or moreand, even upon two-sphere crushing against a steel ball (SUJ-2 ball),the crushing load is 100 kN or more, and both of crushing strengths aregood and vary little, and characteristics are stable.

With the bearing ball according to Example 5, there are no snow flakesobserved by the optical microscope, in the bearing ball and, forexample, no distortion remains inside, so that it is possible to stablysecure the crushing strength and make characteristics of multiplebearing balls even.

Further, FIG. 8 illustrates dimensional accuracy measurement results ofa bearing ball of a 5/32 inch standard.

Samples used for experiments include samples illustrated in FIGS. 5 and6 according to Example 5 of the present invention and examplesillustrated in FIGS. 5 and 6 according to Comparative Example 4, and tensamples of the same manufacturing lot are extracted at random and usedfor examples and comparison examples.

The bearing balls according to Example 5 and Comparative Example 4 arefinally polished and processed such that the sphericity is 0.03 μm, andboth of the bearing balls are finished with substantially stableaccuracy in the range of 0.02 μm to 0.04 μm.

With this sample, the bearing ball according to Comparative Example 4has significantly varying surface roughness (Rmax) of 0.0053 to 0.0127μm, and has unstable accuracy.

With the bearing ball according to Comparative Example 4, irregular snowflakes in the surface layer of the sintered ceramic make wear resistancedifferent from other portions, make the amount of agent greater than agiven amount, increases a thermal expansion difference between siliconnitride crystal and the glass phase, increases inner distortion andincreases unevenness in the composition, thereby preventing improvementin surface roughness upon polishing and processing and making itdifficult to make surface roughness of multiple bearing balls even.

In contrast, the bearing ball according to Example 5 has good surfaceroughness (Rmax) of 0.0040 to 0.0077 μm and little variation, and hasstable accuracy.

With the bearing ball according to Example 5, there are no snow flakesobserved by the optical microscope, and wear resistance of the surfaceis even, so that it is possible to improve surface roughness uponpolishing and processing and make surface roughness of multiple bearingballs even.

By employing the bearing ball using the sintered ceramic according tothe present invention having the above characteristics, for a rollingmember of a sliding device such as a bearing or a ball nut or a valveand the like of fluidic valve which controls a high pressure fluid, itis possible to reduce the weight of a sliding device and a fluidicvalve, prevent damages due to a load and repetitions of sliding ordamages and the like by wear, corrosion and electrical corrosion,maintain performance for a long period of time, increase the life ofcomponents of the sliding device and the fluidic valve and reducemaintenance labor, which are effects originally derived fromcharacteristics of the sintered ceramic. In addition, it is possible toreduce surface peeling due to fatigue resulting from repetitions ofloading, stably provide multiple products after removing variations in,for example, characteristics and accuracy according to lot andvariations in, for example, characteristics and accuracy of individualbearing balls of the same lot and reduce the manufacturing time andmanufacturing cost, and consequently, it is possible to stably makeperformance higher and a life longer, reduce maintenance labor andreduce cost.

Further, dimensional accuracy such as the sphericity and surfaceroughness of the bearing ball improves, so that it is possible tosuppress wear, vibration and noise and cancel an influence on the entiremechanical device of use due to wear and vibration.

In addition, although the sintered ceramic according to the aboveexample is formed in a spherical shape and processed to a bearing ball,the sintered ceramic may be formed in any shape such as a roller shapeincluding, for example, a columnar shape, a circular truncated conicalshape, a barrel shape or a hourglass shape other than the sphericalshape, or a race shape of a bearing, and may be used in any componentsof any devices for which the sintered ceramic can be used.

Example 1

Next, a ceramic-sphere inspection device according to the presentinvention will be described.

As illustrated in FIGS. 9 and 10, a ceramic-sphere inspection device 100according to an example 1 of the present invention has a rotationsupporter which rotatably supports a ceramic sphere S, which is aninspection target, at a given position, alight projector 110 which emitsilluminating light toward the surface of the ceramic sphere S, a lightreceiver 120 which detects light reflected from the ceramic sphere S asinspection light and a processor 140 which evaluates a state of an innerpart of the surface layer of the ceramic sphere S in response to adetection output from the light receiver 120.

The rotation supporter is formed with one driving roller 131 and aplurality of driven rollers 132, and the plurality of driven rollers 132are pivotally supported by a holding member 132 rotatably.

The holding member 134 is supported by a swing supporting unit 133 toswing about a swing shaft 135, so that the ceramic sphere S is rotatablyheld between one driving roller 131 and the plurality of driven rollers132 and the held ceramic sphere S can be released.

The light projector 110 has a light source 111, and a light projectingunit 112 which guides light of the light source 111 as illuminatinglight to a surface of the ceramic sphere S.

The light receiver 120 has a light amount detecting unit 121 and a lightreceiving unit 122 which guides inspection light from the ceramic sphereS to the light amount detecting unit 121.

Front ends of the light projecting unit 112 and the light receiving unit122 form contact surfaces which can contact the surface of the ceramicsphere S, and are held by a proceeding/retreating mechanism, which isnot illustrated, in the holding member 134 to proceed and retreat to andfrom the surface direction of the ceramic sphere S.

The light source 111 only needs to include a wavelength which allowsilluminating light to transmit to the inner part of the surface layerand include the amount of light which is irregularly diffused andspreads in the inner part and reaches the light receiving unit 122, andmay be any light source such as a laser light source which canefficiently output only an optimal wavelength or a halogen light sourcewhich is cheap and has a great amount of light.

When, for example, whether or not there are flaws and snow flakes on thesurface and near the surface is observed, the sintered ceramic accordingto the present invention allows transmission of halogen light up toabout 250 μm, and light having a wavelength between 500 nm and 800 nmcan be observed, so that it is possible to employ, for example, a commonhalogen light source without providing a costly light source such aslaser or a complicated optical system and the like.

Further, the light projecting unit 112 and the light receiving unit 122only need to block light from surfaces other than the contact surfaces,and may use, for example, optical fibers and the like.

The driving roller 131 can be intermittently driven, the contactsurfaces at the front ends of the light projecting unit 112 and thelight receiving unit 122 repeat closely attaching to the surface of theceramic surface S when driving of the driving roller 131 is stopped tooptically observe the inner part of the surface layer and detaching fromthe surface of the ceramic sphere S when the driving roller 131 isdriven.

According to the above configuration, at multiple portions on thesurface of the ceramic sphere S, it is possible to observe onlyilluminating light transmitted and diffused in the inner part of thesurface layer and reflected from the inner part without detecting thelight reflected from the surface.

Consequently, it is possible to accurately detect flaws in the innerpart of the surface layer and snow flakes F formed based on a slightdifference in the composition of a crystal grain boundary phase, as achange of inspection light detected by the light receiver withoutdestroying the ceramic sphere and depending on the surface state, andinspect whether the material of the ceramic sphere is good and theceramic sphere is sintered well by observing the total amount ofinspection light.

In addition, although, with the present example, the light projectingunit 112 and the light receiving unit 122 are integrated and held by theholding member 134 to proceed and retreat to and from the surfacedirection of the ceramic surface S, only one of the light projectingunit 112 and the light receiving unit 122 may be held to proceed andretreat and the light projecting unit 112, and the light receiving unit122 may be fixed in a state where both of them are in contact with thesurface of the ceramic sphere S.

Further, as long as the distance between the front ends of the lightreceiving unit 112 and the light receiving unit 122 is separated suchthat light reflected from the surface is not detected, both of the lightprojecting unit 112 and the light receiving unit 122 may be fixed at aposition at which both of them do not contact the surface of the ceramicsphere S.

When both of the light projecting unit 112 and the light receiving unit122 are fixed, the driving roller 131 may be continuously driven insteadof being intermittently driven.

Further, the ceramic-sphere inspection device 100 is not only applicableto the ceramic sphere according to the present invention and any ceramicsphere as long as ceramic spheres are made of materials which allowtransmission of illuminating light to the inner part of the surfacelayer such as silicon nitride sintered compact made according to othercompositions or other manufacturing methods and ceramics mainly made of,for example, alumina, zirconia and sialon.

When, for example, a halogen light source emits light, materials areknown which allow transmission of light to about 3 to 8 mm in case ofalumina and transmission of light to about 2 to 3 mm in case ofzirconia, and these materials can be inspected.

Second Example

Next, an example will be described where a configuration is employed forobserving the entire surface of the ceramic sphere all over.

With the ceramic-sphere inspection device 200 according to Example 2 ofthe present invention, a rotation supporter is formed with one drivingroller 231 and a plurality of driven rollers 232 similarly to Example 1,and the plurality of driven rollers 232 is pivotally and rotatablysupported by a holding member 234.

Further, as illustrated in FIGS. 11 and 12, a plurality of lightprojecting units 212 and light receiving units 222 are provided over asemicircle of an outer peripheral circle in the cross section passingthe center of the ceramic sphere and at a right angle with respect tothe rotation direction.

Consequently, by rotating the ceramic sphere S once, it is possible toobserve the entire surface and efficiently inspect the ceramic sphere S.

In addition, although the projecting units 212 and the light receivingunits 222 are drawn bold for ease of understanding, a greater number ofthe projecting units 212 and the light receiving units 222 can beprovided using thin components such as optical fibers more than theillustration.

Further, the number of light projecting units 212 may be less than thenumber of light receiving units 222.

Example 3

As illustrated in FIG. 13, with, a ceramic-sphere inspection device 300according to Example 3 of the present invention, a rotation supporter isformed with one driving roller 331 and a plurality of driven rollers332, and the plurality of driven rollers 332 are pivotally supported bya holding member 334 to rotate around parallel rotation shaft lines C2and the driving roller 331 is rotatable around a rotation shaft line C1inclined at a given angle θ from the rotation shaft lines C2 of theplurality of driven rollers 332.

Further, a plurality of light projecting units 312 and light receivingunits 322 are provided along an outer peripheral circle in the crosssection passing the center of the ceramic sphere S and at a right anglewith respect to the rotation direction.

The inclination angle θ of the rotation shaft line C1 of the drivingroller 331 is set to be shifted by the width of the plurality of lightreceiving units 322 when the ceramic sphere S rotates around.

Although it is necessary to rotate the ceramic sphere S a plurality oftimes to observe the entire surface, it is possible to observe theentire surface using a less number of light receiving units 322 than inExample 2.

In addition, although the light projecting units 312 and the lightreceiving units 322 are drawn bold for ease of understanding similarlyto Example 2, a greater number of the projecting units 312 and the lightreceiving units 322 can be provided using thin components such asoptical fibers more than the illustration.

Further, the number of light projecting units 312 may be less than thenumber of light receiving units 322.

Further, as illustrated in FIG. 14, the number of the light receivingunit 312 and the number of the light receiving unit 322 may be one,respectively, so that the inclination angle θ of the rotation shaft lineC1 of the driving roller 331 becomes slight.

Consequently, an operation of adjusting the light amount and thesensitivity of a plurality of light projecting units and light receivingunits is not required at all, so that it is easy to maintain inspectionaccuracy.

In addition, the rotation supporter is by no means limited to the aboveexamples and may employ any configuration as long as it supports theceramic sphere S rotatably at a given position according to the sameoperation as in Examples 1 to 3.

For example, as illustrated in FIG. 15, two opposing driven rollers 432which rotatably support the ceramic sphere S may have conicaltrapezoidal shapes having rotation shafts 437 extending in alongitudinal direction. According to this configuration, by decenteringgears 436 by a decentering amount r to attach to the rotation shafts 437of the two opposing driven rollers 432 and cyclically fluctuatingrotation speeds of the two driven rollers 432, it is possible to rotatethe ceramic sphere S, and observe the entire surface of the ceramicsphere S all over with twisting movement.

Further, as illustrated in FIG. 16, driving rollers 531 are formed inshaft shapes having parallel grooves 538 and screw grooves 539 tocontinuously inspect a plurality of ceramic spheres S.

With the embodiment illustrated in FIG. 9, a plurality of parallelgrooves 538 extending in a circumferential direction over about asemicircle of the shaft-shaped driving rollers 531, and a plurality ofscrew grooves 539 extending to connect to the parallel grooves 538adjacent in the other semicircle continuing to the parallel grooves 538are formed.

Further, light receiving units 522 are provided to meet the parallelgrooves 538, respectively, and the ceramic spheres S are rotated in theparallel grooves 538 at the position of the light receiving units 522,and moved by the screw grooves 539 in the shaft direction of the drivingroller 531 to change the rotation shaft.

By this means, the ceramic spheres S are continuously introduced fromthe left direction in FIG. 16, are carried to the parallel grooves 538sequentially in the right direction, and are observed while the rotationshafts are sequentially changed by the light receiving units 522 meetingthe parallel grooves 538 respectively to continuously observe the entirespherical surface.

In addition, as illustrated in FIGS. 15 and 16, in another embodiment ofa rotation supporter, the numbers of light receiving units and lightprojecting units may be one or plural, and, further, the driving rollermay be intermittently driven, and the light receiving units and thelight projecting units may proceed and retreat to and from the surfacedirection of the ceramic spheres S and closely attach to the surfaces ofthe ceramic spheres S only when the driving roller stops.

REFERENCE SIGNS LIST

-   100, 200, 300 CERAMIC-SPHERE INSPECTION DEVICE-   110 LIGHT PROJECTOR-   111 HALOGEN LIGHT SOURCE-   112, 212, 312 LIGHT PROJECTING UNIT-   120 LIGHT RECEIVER-   121 LIGHT AMOUNT DETECTING UNIT-   122, 222, 322, 522 LIGHT RECEIVING UNIT-   131, 231, 331, 431, 531 DRIVING ROLLER-   132, 232, 332, 432 DRIVEN ROLLER-   133 SWING SUPPORTING UNIT-   134 HOLDING MEMBER-   135 SWING SHAFT-   436 GEAR-   437 ROTATION SHAFT-   538 PARALLEL GROOVE-   539 SCREW GROOVE-   S CERAMIC SPHERE-   F SNOW FLAKE F-   S1, S2 MEASUREMENT TARGET SPHERE-   510, 520 ANVIL

1. A sintered ceramic which has excellent wear resistance and which isobtained by molding and sintering a mixture comprising silicon nitrideand a sintering agent made of AI₂O₃ and Y₂O₃, wherein a bulk density is3.1 g/cm³ or more, an average grain size is 3 μm or less, and there areno flaw of 10 μm or more from a surface to a depth of 250 μm and nowhite spot (snow flake) of 20 μm or more.
 2. A ceramic sphere which hasexcellent wear resistance and which is obtained by molding and sinteringin a spherical shape a mixture comprising silicon nitride and asintering agent made of Al₂O₃ and Y₂O₃, wherein a bulk density is 3.1g/cm³ or more, an average grain size is 3 μm or less, and there are noflaw of 10 μm or more from a surface to a depth of 250 μm and no whitespot (snow flake) of 20 μm or more.
 3. A wear resistant member formedusing a sintered ceramic, wherein a surface of the sintered ceramicaccording to claim 1 is processed to make a rolling bearing member.
 4. Aceramic-sphere inspection device comprising: a rotation supporter whichrotatably supports the ceramic sphere according to claim 2 at a givenposition; a light projector which emits illuminating light toward asurface of the ceramic sphere; a light receiver which detects lightreflected from the ceramic sphere as inspection light; and a processorwhich evaluates a state of an inner part of a surface layer of theceramic sphere in response to a detection output from the lightreceiver, wherein the light receiver does not detect the illuminatinglight emitted from the light projector and reflected at the surface ofthe ceramic sphere.
 5. The ceramic-sphere inspection device according toclaim 4, wherein the light projector includes: a light source; and alight projecting unit which guides light of the light source to thesurface of the ceramic sphere as illuminating light, the light receiverincludes: a light amount detecting unit; and a light receiving unitwhich guides the inspection light from the ceramic sphere to the lightamount detecting unit, and at least one of the light projecting unit andthe light receiving unit includes at a front end a contact surface whichcan contact the surface of the ceramic sphere.
 6. The ceramic-sphereinspection device according to claim 4 or claim 5, wherein a pluralityof light receiving units are provided over a semicircle of an outerperipheral circle in a cross section passing a center of the ceramicsphere, and the rotation supporter rotates the ceramic sphere at a rightangle with respect to the outer peripheral circle on which the lightreceiving units are provided.
 7. The ceramic-sphere inspection deviceaccording to claim 4 or claim 5, wherein a plurality of light receivingunits are provided along part of an outer peripheral circle in a crosssection passing a center of the ceramic sphere, and the rotationsupporter rotates the ceramic sphere at a right angle with respect tothe outer peripheral circle on which the light receiving units areprovided, and rotates the ceramic sphere in an outer peripheral circledirection on which the light receiving units are provided such that theceramic sphere is shifted by a width of the plurality of light receivingunits when finishing rotating round.
 8. The ceramic-sphere inspectiondevice according to claim 4 or claim 5, wherein only one light receivingunit is provided, and the rotation supporter rotates the ceramic spherein a given direction, and slightly rotates the ceramic sphere at a rightangle with respect to a rotation direction.
 9. The ceramic-sphereinspection device according to claim 8, wherein a same number of lightprojecting unit as the number of the plurality of light receiving unitsis provided, and each one of the light projecting units is providedadjacent to each of the plurality of light receiving units.
 10. Theceramic-sphere inspection device according to claim 9, wherein therotation supporter can be intermittently driven; at least one of thelight projecting unit and the light receiving unit can proceed andretreat to and from a surface direction of the ceramic sphere, and acontact surface at a front end of at least one of the light projectingunit and the light receiving unit closely attaches to the surface of theceramic sphere when driving of the rotation supporter is stopped, and isdetached from the surface of the ceramic sphere when the rotationsupporter is driven.